The present invention concerns a method as well as a system for determining the movement of a ship.
For successful navigation within minimum associated risk it is necessary to be informed of all ship movements that occur. Ship movement may be the result of self-derived forces generated by one or more propellers and by one or more rudders, or they may be the result of forces due to wind, waves and current. Irrespective of their cause, it is important that the navigator is informed of all movement in order to be able to accomplish successful maneuvers with the ship, such as approaching a harbor, berthing the ship along a jetty, approaching and contacting another ship or a single point mooring and/or anchoring.
Systems for determining ship movements are known, however, none of these provide sufficient and accurate information under all the above-mentioned circumstances.
In one such known system use is made of two master radars, which are placed on a jetty and connected to a computer. These radars cooperate with two transponders onboard the ship at accurately known positions.
The master radars transmit interrogating pulses in turn on the same frequency, the first transponder replies on a second frequency and the other transponder on a third frequency. In this way, the radars can be used to measure the ranges between the radars and each of the transponders and these measurements fed into the computer make it possible to calculate the bow range of the ship, its stern range together with the velocities and accelerations. In addition, the computer calculates the angle .phi. which the center line of the ship makes with that of the jetty and the rate of the change of the angle .phi..
This system is restricted in its application as it can only be used at those locations where master radars are present. It is not able to provide ship movement information in open ocean conditions neither the vital information required during transits through hazardous channels affected by strong and varying cross-currents and winds.
In one other system, use is made of two acoustic transducers located through the bottom hull of the ship, one at each end. Theoretically, this system has the advantage of being independent of any device external to the ship but in practice, it is found that hull and/or propeller induced aeration and/or cavitation beneath the stem of the ship causes the aft located transducer to fail under certain circumstances and particularly when maneuvering. Also, it is costly to install two transducer assemblies.